The effects of large scale agriculture on soil health

There are 13 billion hectares of soil on earth but only 1.4 billion can be used for agriculture. The rest are in some way unfit for use such as too rocky, too steep or contaminated. Over a third of all arable land is used for farming so the effects of intensive agriculture on soil should be examined. (Kelley, 1990). Soils have many ecological functions such as:
-supply nutrients
-anchor plants/ physical support
-habitat for animals/ soil biota
-retain moisture/drainage
-buffer temperature and pH changes
-filter and regulate water flow to groundwater/detoxification
-nutrient cycling/decomposition
The question however remains as to the consequences of modern agricultural practices on soils ecological functions.

Soil erosion and lose of arable land

The degradation of farm land is of huge concern. The major form of degradation is through erosion of the topsoil. This is caused by natural forces such as wind and water but is aggravated when soil is ploughed and then left bare. Roots of plants normally hold much of the soil in place against theses elements. However loose dry soil can be blown more easily by wind. Stripped soil also cannot hold or absorb water as well and is more easily washed away by the force of water or rainfall. Soil washes away into water sources such as river and lakes as sediment. The amount of topsoil lost is 75 billion tonnes per year or 17 tonnes per hectare per year (Pimentel 1995). In the last 150 years approximately half of the planets topsoil has been lost (Soil erosion and degradation, n.d.). The topsoil contains most of the biological activity, nutrients and organic matter. A loss of topsoil leads to poorer soil structure, water holding capacity, nutrient retention, changes in pH and lower resistance to drought (Kelley, 1990). All of this means there is a loss of fertility in the soil. It is estimated that this costs the United States $20million per year in just the nutrients lost due to erosion (Understanding soil erosion in irrigated agriculture, n.d.). Tillage can also cause compaction. Although it loosens the very surface of the soil there is a layer underneath that becomes compacted. Compaction can also be caused by traffic on the soil, rainfall and minimal crop rotation. Compaction decreases yields, causes loss of nitrogen, reduces availability of potassium, reduces aeration and decreases water infiltration (Hughes et al.). Another form of soil degradation is desertification which is the degradation of drylands. This occurs mostly due to over use of groundwater, tillage from agriculture and overgrazing. About half of all land is dryland and about 10-20% of all drylands have suffered from desertification (Hogan, 2010). Consequences of desertification are loss of vegetation, reduction of biodiversity and formation of dust clouds. Desertification can also lead to erosion (Desertification, n.d.).

Pollution/Soil contamination

The main sources of pollution from agriculture in terms of soil and water are pesticides, fertilizers and runoff, which can include synthetic chemical, sediment from erosion and waste products. Pesticides don’t just kill pests they also harm soil biota and can also effect the health of plants. By harming the soil biota they reduce the biodiversity of the soil. They can also negatively impact plant roots systems (The problem with pesticides, n.d.). Pesticides can also enter water ways and cause more harmful effects (Conway & Pretty, 1991). A common agricultural fertilizer ammonium nitrate can break down into ammonium and nitrate. The nitrates can leach through soil as well when mixed with water it can become slightly acidic thus affecting pH. Fertilizers can lead to acidification of the soil from long term use. High levels of fertilizers can also affect association of roots with mycorrhizal fungi (Fertilizers, n.d.). They also do not replenish trace nutrients. When fertilizers leach into water sources they can cause algae blooms. Heavy metals can also be found in some fertilizers such as lead, mercury, cadmium and arsenic. Sediment from erosion of agricultural lands can get into and change the dynamics of water systems. Manure from livestock farming can leach nitrate if not stored and disposed of properly. This can have a similar effect as fertilizers and cause algae blooms (Agricultural pollution, n.d.). Animals are also fed antibiotics and other drugs and these chemicals can be excreted in their waste and be another source of contamination. The effect of this on ecosystems is not yet known.

Water and Irrigation

70% of all fresh water is used for agriculture (Water uses, n.d.). The quality of the water being applied to the soil and leaving the soil as well as the effects of it on the soils should be considered. Over irrigation can lead to the depletion of underground aquifers and water logging. Water logging of soils can lead to gleying which can then lead to poor crop production. Water logging is more common in soil with heavy clay and poor drainage (Water logging and inundation why they could be costing you money, n.d.). Excessive pumping of water from water tables causes salts and chemicals to leach back down to the source and contaminate the water table (Letey, 2000). High evaporation can lead to increased salinity which is often due to under irrigation. All water contains some dissolved salts in it and as water evaporates it leaves behind these salts (Letey, 2000). Bare soils are more likely to have higher evaporation rates leading to loss of soil moisture (Soil moisture and temperature considerations, n.d.). Water in soils can help buffer temperature changes. During high temperatures evaporative cooling allows the soil to regulate its own temperature and keep it at a reasonable temperature (How soils send messages on heat waves, n.d.). Low moisture contents in soils can have a negative impact on plant growth and thus crop yields. Waste water and runoff can lead to pollution (see Pollution).
Soil Biota

Soil biota play a huge role in nutrient cycling, the break down of organic matter and they help improve the structure of the soil. Agricultural practises can have effects on soil biota. Pesticides can hurt soil biota (see Pollution). Erosion of topsoil and organic matter can lead to a loss of soil organisms in that horizon and food for those soil organisms. Water logging of soils can lead to gleying which can lead to anaerobic condition which can have a negative impact on positive soil biota (Irrgation/Water). The conversion of natural ecosystems to farm land as well as the intensification of crop production through monocropping and synthetic fertilizers has a negative impact on soil biota. Larger soil biota are more negatively impacted than smaller organisms, however smaller organisms are affected more from long term intensive practises (Postma-Blaauw et al., 2010). Overall soil biota are negatively affected by intensive agricultural practices.

Conclusion

Large scale conventional agricultural practices have a vastly negative impact on soil. The erosion of usable topsoil contributes to a physical loss of matter as well as depleting nutrients and causing contamination of waterways. This takes away from the physical support for plants as well as essential nutrients, especially trace nutrients. Compaction reduces pore space which affects aeration, water retention and drainage. These effects are then detrimental to soil biota. Pollution can occur by leaching of fertilizer or pesticides into water systems or by sediment from erosion. This can cause disruptions to waterway systems in both physical clogging and biological in terms of algae blooms. This means that the soil is not performing its function of filtering, regulating and detoxifying the flow of water to the groundwater. Pesticides can harm the soil biota and reduce biodiversity. Salts from fertilizers can affect the pH of soil. Over and under irrigation can cause drainage issue such as gleying or loss of moisture through evaporation and lead to salinity. Which can also lead to pH changes. As well water content in soil plays a role in temperature regulation so incorrect irrigation practises can affect the soils ability to perform this function. Intensive agricultural practices have been shown to harm soil biota especially larger organisms in the short term but also affecting smaller biota in the long term. Biota are largely responsible for nutrient cycling and decomposition so where soil organisms are compromised so is the soils ability to perform this function. Overall the effects of agriculture on soils is extremely detrimental and an alternative methods of farming should be explored.